Inorganic chemistry,spherical aromaticity is formally used to describe an unusually stable nature of some spherical compounds such asfullerenes and polyhedralboranes.
In 2000, Andreas Hirsch and coworkers inErlangen,Germany, formulated a rule to determine when a spherical compound would bearomatic. They found that those with 2(n+1)2 π-electrons could display aromatic properties, as spherical molecular orbitals are filled when there are 2(n+1)2 π-electrons for some positive integern. For example, inbuckminsterfullerene (C60) this happens for the species C6010+, which has 50 π-electrons: 50/2 = 25, which is aperfect square.[1]
In 2011, Jordi Poater and Miquel Solà expanded Hirsch's rule to open-shell spherical compounds, which have unfilled outer shells but are still aromatic. They found that spherical compounds with 2n2+2n+1 π-electrons withspin S = (n + 1/2) would also display aromatic properties, sometimes more aromatic than comparable closed-shell species. This corresponds to the outer shell being half-filled,[2] and is similar toBaird's rule. For example buckminsterfullerene with one additional electron, (C601–) is aromatic, with S = 11/2 and a bond-length alternation of 0.2pm.
